This invention relates to optical communication systems and, more particularly, to apparatus for converting a bipolar electrical signal into an optical signal capable of being transmitted over an optical transmission system.
Large numbers of messages are now transmitted over the telephone plant by means of T1 and T2 Carrier Systems. In these systems, speech signals are coverted into bipolar electrical signals which are essentially binary in nature but the adjacent logic "1s" are caused to alternate in polarity. This alteration in polarity was deemed necessary in order to insure that a sufficient number of transitions would be available in the signal in order to permit the repeaters to derive timing information and to provide dc balance to preclude baseline wander in the ac coupled receiver. In addition, violations in the alternating polarity, known to those in the art as "bipolar violations," are inserted in order to stress receivers by introducing known amounts of baseline wander. The medium used for connecting the terminal stations and repeaters is wire pair cable.
A large number of wire pair cables utilized to transmit both T1 and T2 carrier signals have already been installed in the major cities. These cables are physically positioned within ducts beneath the surface of the streets of the cities. Many of the ducts have already been loaded with wire pair cables to their full capacity. Expansion of the telephone plant in these areas, if that expansion is to take place with similar T Carrier Systems, will require the installation of additional ducts. It would be advantageous if the existing wire pair cables in these ducts could be replaced by optical fibers inasmuch as each fiber is smaller than a wire and, in addition, may allow larger bandwidths to be transmitted, thus increasing the total number of conversations which may be sent within any given duct.
In the period of transition when wire pair cables are being replaced by optical fibers, many electrical bipolar signals of the type generated in the T1 Digital Transmission System will have to be converted into optical signals in order to permit them to be transmitted over optical fibers. Inasmuch as there is no straightforward equivalent of two polarities in the optical signal, some sort of conversion is necessary. It would also be advantageous if the resulting optical signal were simply of the binary type as opposed to a multilevel optical signal, in order to simplify the repeater units which will be necessary in the optical transmission system. Finally, it is desirable to maintain the polarity information present in the bipolar signal of the T Carrier System inasmuch as polarity transitions and bipolar violations will continue to provide information to T carrier type equipment operating at the receiving end of the optical transmission system.